Having had a very bad experiences with letting agencies in the past, I would never use one again. In my experience, they are only interested in letting the property and getting the fee, after that, they don't really care. Even the ones that go on to manage the property aren't terribly interested in maintenance etc.
I prefer to meet the tenants and try to get a feeling of how they are and check the references myself.
I also use trusted tradesmen and leave their numbers with the tenants in case they have a problem.
In a nutshell, you would be better off letting the property yourself before you go abroad, have the rent paid directly into your bank account and monitor it closely online and have a relative on standby in case there's a problem with the property.
Don't forget to register with the PRTB and to inform your insurance company that you are letting the property (you'll need landlord's liability insurance).
Good luck with the letting and the new life abroad!